1. Field of the Invention
The present invention relates to a method and an apparatus for producing a semiconductor device which is capable of obtaining a crystalline silicon film high in uniformity by improving the flatness of a glass substrate during producing an insulated gate semiconductor device such as a thin film transistor (TFT) formed using a non-single crystal silicon film which is formed on the substrate, or other semiconductor devices. In particularly, the present invention is useful in producing a semiconductor device formed on the glass substrate.
2. Description of the Related Art
In recent years, several researches have been made into insulated gate field effect transistors having a thin film-shaped active layer (so-called active region) on an insulated substrate, which are so-called TFT).
Those TFTs are classified, for example, into an amorphous silicon TFT and a crystalline silicon TFT, depending upon the material or the crystal state of a semiconductor as used. The crystalline silicon stated here is directed to non-single crystal silicon which is not single crystal. Hence, those TFTs are generally called "non-single crystal silicon TFTs".
In general, the electric field mobility of a semiconductor which is in an amorphous state is small, and therefore not available to the TFTs that require a high speed operation. Also, the amorphous silicon cannot be used to produce a p-channel TFT (PMOS TFT) since the p-type electric field mobility of the amorphous silicon is remarkably small, and thus a complementary MOS circuit (CMOS) cannot be formed by the combination of the p-channel TFT and an n-channel TFT (NMOS TFT).
The crystalline semiconductor is larger in the electric field mobility than the amorphous semiconductor, and therefore enables a high speed operation. The crystalline silicon can be used for obtaining not only the NMOS TFT but also the PMOS TFT, thereby forming a CMOS circuit.
A crystalline silicon film is obtained by thermally cooling an amorphous silicon film obtained through the vapor phase growth technique at an appropriate temperature (600.degree. C. or higher) for a long period of time, or by irradiating an intense light such as a laser beam (optically annealing).
However, in the event of using a glass substrate which is inexpensive and rich in processability as an insulating substrate, it is extremely difficult to obtain, by only annealing, a crystalline silicon film which is satisfactorily high in electric field mobility (high to the degree that the CMOS circuit can be formed).
This is because the above glass substrate is generally low in strain point (about 600.degree. C.), and thus when the temperature of the substrate is elevated up to a temperature required for obtaining a crystalline silicon film sufficiently high in mobility, the substrate is warped.
In the event of applying the optically annealing technique for crystallizing a silicon film formed on the glass substrate, a high energy can be applied only to the silicon film without elevating the temperature of the substrate so much. Hence, the optically annealing technique is very effective in crystallizing the silicon film formed on the glass substrate.
It has been found that a high power pulse laser such as an excimer laser is the most suitable for the optimum light source for optically annealing. The maximum energy of the laser is extremely larger than that of the continuously oscillating laser such as an argon ion laser, and the mass productivity could be enhanced using a large spot which is several cm.sup.2 or more in size.
However, because the beam as normally used is square or rectangular in shape, the beam is required to move vertically and horizontally for processing a single substrate having a large area. Thus, the optically annealing technique needed to be still improved from the viewpoint of the productivity.
The above matter could be remarkably improved with a technique in which the beam is deformed linearly, the width of the beam is a length that exceeds that of a substrate to be processed, and the substrate is scanned relatively by the beam (The scanning operation in the specification means that linear laser beams are irradiated onto the substrate while being overlapped one on another bit by bit.). The details are disclosed in Japanese Patent Unexamined Publication No. 5-112355.
The silicon film which is still high in crystallinity can be prepared by conducting the thermally annealing process prior to conducting the optically annealing process. In the thermally annealing process as described in Japanese Patent Unexamined Publication No. 6-244204, utilizing the effect that an element such as nickel, iron, cobalt, platinum, paradium (hereinafter referred to as "crystallized catalytic element, or simply "catalytic element") promotes the crystallization of amorphous silicon, the crystalline silicon film can be obtained by the thermally annealing process at a lower temperature for a shorter period of time in comparison with the normal case.
TFTs arranged in the form of a matrix are formed with a crystalline silicon film formed using the thermally annealing process and the optically annealing process together, and the distribution of their threshold voltage in the substrate surface is investigated.
FIG. 2 shows the distribution of the threshold values of the TFT using the crystalline silicon film formed through the conventional method, within the substrate surface. The distribution is U-shaped as shown in FIG. 2.
FIG. 4 shows the arrangement of TFTs on the glass substrate. The data in FIG. 2 is obtained, as shown in FIG. 4, in such a manner that the TFTs of 400.times.300 pieces are arranged in the form of a matrix in a region of 40.times.50 mm on a Corning 1737 substrate of 100 mm.sup.2, and the respective locations of 400 TFTs disposed laterally in a line from one end to the other end (a portion surrounded by a dotted line in FIG. 4) are indicated correspondingly in the axis of abscissa.
When the pixel matrix that constitutes the pixel portion of a liquid crystal display has the distribution of threshold voltages shown in FIG. 2, the display state becomes nonuniform, resulting in a defective image.
As a result of researching the cause that the threshold voltage exhibits such a U-shaped distribution within the substrate surface by the applicant, it has been found that a tendency of the U-shaped distribution is very similar to the warp of the substrate immediately before a laser beam is irradiated onto the substrate.
Also, no warp of the substrate is found in the glass substrate immediately after an amorphous silicon film is formed on the glass substrate, and it has been found that the warp of the substrate is caused because, during a heat treatment (by which the film grows in the solid phase into a crystallized film) subsequent to the amorphous silicon film forming process, a silicon film (or silicon oxide film) is contracted higher than that of the glass substrate in cooling the substrate after the heat treatment. The warp of the substrate is produced in a U-shape viewed from the film formed on the substrate.
FIG. 3 shows a state in which a laser annealing is conducted on the silicon film formed on the substrate which has been warped. From FIG. 3, when the laser annealing is conducted on the silicon film in such a state where the substrate is warped, the focal point of the laser beam is shifted differently at the respective locations on the substrate.
It is presumed that the shift of the focal point makes the crystallinity of the silicon film different from each other within the substrate surface, so that the threshold voltage exhibits a specified distribution within the substrate surface.
The warp of the substrate immediately before a laser beam is irradiated onto the substrate having 100 mm square is different by about 50 .mu.m between the central portion and the edge portion of the substrate. The degree of the warp fell within a range of about 20 to 200 .mu.m although it depends upon the temperature of the above heat treatment process, a time necessary for processing, the material of the substrate, or the like. There is a case in which when the size of the substrate is about 500 mm square, its warp becomes about 1 to 2 mm.